1. Field of the Invention
The present invention relates to a coated optical fiber prepared by coating an optical fiber with first and second layers.
2. Description of the Prior Art
An optical fiber utilized for optical communication systems has a small outer diameter of generally 100 .mu.m order and is a very fragile material. Thus, if such optical fibers are used without any reinforcement, they rupture at a strength which is very much lower than the original strength (about 7 kg) thereof, because of friction or generation of surface damage during the fabrication or cable-manufacturing process. Namely, it is not possible to form a transmission line with a high reliability. In this connection, there is proposed a method for coating the surface of an optical fiber immediately after spinning thereof with a plastic material in order to protect the surface of the optical fiber and to maintain the initial strength thereof.
Such plastic coating consists generally of a primary coating layer and a secondary layer. The primary coating layer is a buffer layer made of a material of a low Young's modulus and is intended to maintain the initial strength of the optical fiber and prevents an increase in the microbending loss of the fiber due to an uneven secondary coating. On the other hand, the secondary coating layer is made of a thermoplastic resin having a higher Young's modulus than that of the primary coating layer and has as its purpose to easily handle optical fibers in cabling or the like process therefor.
Heretofore, there have been proposed the following two types of coated optical fibers. One of the two types is a coated optical fiber having a tight construction in which a primary coating layer made of a thermosetting resin such as silicone resin or the like adheres tightly to a secondary coating layer made of a thermoplastic resin such as nylon, polyamide resin or the like. Another type is a coated optical fiber of the loose tube type having such a construction that a primary coating layer made of a thermosetting resin such as acrylic resins or the like is loosely held in a protective plastic tube (secondary coating layer) made of a thermoplastic resin such as polyethylene terephthalate, polypropylene or the like.
In either of the aforesaid tight and loose tube types of coated optical fibers, it is required that the primary coating layer adheres to the surface of an optical fiber in order to protect the optical fiber. Either of thermosetting and thermoplastic resins is applicable to the coating of an optical fiber as described above. However, the materials which have heretofore been industrially employed are limited to thermosetting resins such as silicone, urethane-acrylate, epoxy-acrylate or the like resins. For the sake of coating an optical fiber with a thermosetting resin as described above, an optical fiber immediately after spinning is coated with such a resin by means of an applicator such as a die or the like, and the resin thus coated is cured by heating or ultraviolet-ray irradiation.
However, such primary coating material of the thermosetting resin composition as described above and the coating method utilizing the coating material involve some problems from a viewpoint of economization of optical fiber cables. The reasons follows. The first disadvantage resides in that the coating materials are expensive. For example, silicone resin which has been widely used as a primary coating material is more expensive by one figure than that of a thermoplastic resin which is generally used. The second problem resides in that when the spinning rate of the optical fibers increases, slip arises between the coating materials and the optical fibers. Besides, these coating materials require two processes of coating and curing. As a consequence, the optical fibers cannot be coated with such coating materials. In other words, use of these coating materials is limited to a range wherein the coating rate thereof is comparatively low (100 m/min. or less in case of industrial fabrication). Furthermore, the third problem resides in that the transmission loss of an optical fiber coated with such type of materials as mentioned above increases at a low temperature of -60.degree. C. or less, because of an increase of Young's modulus in a material for the primary coating layer in the vicinity of its crystallizing temperature or glass transition temperature.
On the other hand, in the case in which a nylon layer is used as a secondary coating layer, the transmission loss increases due to orientation relaxation or recrystallization of such nylon layer after the nylon layer is heated at a high temperature of +80.degree. C. or more.
As primary coating materials other than those described above, thermoplastic resins such as ethylenevinyl acetate copolymer (EVA) and the like have been proposed in combination with a hot-melt method wherein a die for the coating operation is heated to melt the EVA. In this case, however, there are the same problems as those of the materials described above. Besides, the latter materials and such hot-melt method are not suitable for coating optical fibers of continuous length at a high speed, since the heating temperature is limited by the heat stability of the resin used.
As a method for coating a wire material with thermoplastic resin, use is generally made of an extrusion coating method in which an extruder is utilized in a technical field of coating metal wires such as those used for power lines, communication lines or the like. As a die-nipple construction of this case, there are two types, i.e., the pressure type construction shown in FIG. 1 and the tube type shown in FIG. 2.
More specifically, FIGS. 1 and 2 are typical views each showing a construction of a conventional die-nipple wherein reference numeral 1 designates a glass fiber, 2 concentrically disposed thermoplastic rubber composition and thermoplastic resin in the molten state, 3 a die, 4 a nipple, 5 the inner diameter of the nipple 4, 6 the distance from the extreme end of the nipple 4 to an outlet of the die 3, 7 the inner diameter of the die 3, and 8 the nozzle length of the die 3.
The pressure type means a construction wherein a metal wire is coated with a thermoplastic resin inside a die. The pressure type has advantages in that a coating layer adhering to metal wires is obtained and that high-speed coating of the wire is realized by controlling the pressure of the resin used.
On the other hand, in the tube type, a metal wire is coated with a tube-shaped resin which has been extruded while applying drawdown. The advantages of the tube type resides in that the thickness of the coating film is easily controlled and the coating is performed at a high-speed while maintaining a comparatively low coating tension
In these die-nipple constructions, however, glass fibers cannot directly be coated with a thermoplastic resin. A major reason resides in the difference between the breaking characteristics of metal wires and glass fibers. More specifically, while breaking strength does not decrease in the case of metal wires even if they are in contact with a solid material, the breaking strength of glass fiber decreases easily if the glass fiber comes into contact with the inner surface of a nipple Thus, in the case of coating a glass fiber, it is necessary not only to make the clearance defined between a fiber and the inner surface of a nipple wider (usually, by several tens .mu.m) than that in the case of coating a metal wire, but also to keep the coating tension as low as possible As a result, the coating tension increases due to the generation of excessive resin pressure, i.e., fiber breaking arises as well as back flow of the resin because of the high pressure through the inside of the nipple in this die-nipple construction of the pressure type. Accordingly, it is not possible to obtain a coating layer having a stable film thickness over a continuous length thereof at a rate of several tens m or more per minute.
On one hand, in the tube type construction, it is difficult to obtain a coating layer adhering to a fiber. Besides, such fibers vibrate, since a tube-shaped molten resin thus extruded does not effectively maintain the fiber centered (centering force), so that the fiber comes easily in contact with the inner surface of the nipple. For this reason, fiber strength decreases and in addition, the coating cannot be processed at a high speed.
As described above, there was a problem in economization for the fabrication of an optical fiber when the primary coating material of a thermosetting resin and the coating method which have heretofore been employed for the fabrication of a metal wire are used for the fabrication of an optical fiber. Furthermore, there was also such problem that a thermoplastic resin cannot be used for such primary coating material, since the extrusion coating method which has been utilized for metal wire coating cannot be applied to the coating of a fiber.
On the other hand, concerning the secondary coating layer, the linear expansion coefficient of materials used for both the tight and loose types are of the order of 10.sup.-4 .degree. C..sup.-1 and such a value is far larger than that of fiber itself which is of the order of 10.sup.-7 .degree. C..sup.-1. For this reason, in case of a tight coating fiber, the fiber causes microbending because of expansion and contraction of the secondary coating layer due to temperature change so that there is an increase in loss due to the microbending. Furthermore, in such a coated fiber of the tight type, a comparatively long cooling step of about several meters is required in the step for secondary coating. Such cooling step is performed so that the orientation in fibers along the longitudinal direction thereof which occurs in the extrusion coating step for secondary coating material is removed as much as possible by means of slow cooling. If such slow cooling is insufficient, relaxation of orientation proceeds even at ordinary temperatures so that the secondary coating layer contracts gradually. As a result, compressive strain is applied to fibers, and consequently microbending loss increases gradually. As the secondary coating step is speeded up, it becomes actually impossible to provide such a slow cooling step which can sufficiently correspond to such high-speed secondary coating step. In these circumstance, relaxation in orientation of such secondary coating layer causes a bottleneck so that the coating speed is limited to about several tens m/min Thus, it has been a problem that the coating speed cannot be increased as described above.
As another tight type coating fiber, there has been proposed a coated fiber in which glass fibers are appended longitudinally to fiber yarn stock having a silicone buffer layer along the length thereof, and they are fixed by curing a thermosetting resin applied thereon to form a secondary coating layer. The linear expansion coefficient of the secondary coating layer of the fiber core wire is of the order of 10.sup.-5 .degree. C..sup.-1 and therefore, increase in microbending loss is remarkably suppressed. In this case, however, such a low linear expansion coefficient of the secondary coating layer is due to the linear expansion coefficient (in the order of 10.sup.-7 .degree. C..sup.-1) of glass fiber, whilst the thermosetting resin itself has inevitably a high linear expansion coefficient (in the order of 10.sup.-4 .degree. C..sup.-1). In addition, since thermosetting resin requires a comparatively long curing time, such a disadvantage that secondary coating speed is very slow still remains.
In a coated fiber of the loose tube type, loss due to macroscopic fiber bending caused by expansion and contraction of a protective plastic material forming a secondary coating layer is relaxed by affording suitably an extra length to the fiber in the loose tube. However, since a difference between linear expansion coefficients of the secondary coating layer and the fiber itself is significant, increase in microbending loss due to expansion and contraction of the secondary coating layer is still observed.
In order to prevent the increase in microbending loss due to a difference between the linear expansion coefficients of the secondary coating layer and the fiber, there has been proposed a coated fiber prepared by stretching and orientating a loose tube along the longer direction thereof at its melting point or less under solid state in the core wire manufacturing step. The linear expansion coefficient of the secondary coating layer of the coated fiber is 10.sup.-5 .degree. C..sup.-1 or less and hence, increase in microbending loss is remarkably suppressed. There are, however, such disadvantages that a comparatively long heating oven is required for the stretching and orientation of loose tubes in order to fabricate a coated fiber of the loose tube type which is stretched and orientated as described above, that its production line becomes long, because it is required to place a heat-treating oven after a stretching-heating oven in order to prevent heat shrinkage of the stretched loose tube at a high temperature, that for this reason, it is difficult to increase the manufacturing speed, and that precise control for the manufacturing steps is necessary for controlling the extra length of the fiber.
As mentioned above, a thermoplastic resin having a lower cost than that of thermosetting resins may be used as a secondary coating material, and the above-mentioned extrusion coating may be adopted for the coating method therefor. In this case, however, it is premised on the use of a thermosetting resin as its primary coating material, so that the primary and secondary coating steps become inevitably separate steps regardless of continuous or discontinuous steps. In view of the above, economization of optical fibers based on an increase of the coating speed is limited in the coating techniques at present.
As set forth above, there were such disadvantages in the prior art that the microbending loss based on a difference between the linear expansion coefficients of the coating material and the fiber increases in both coating structures of the tight and loose tube types, so long as secondary coating materials (thermoplastic resins) which are employed at present are utilized, and further that its coating speed is limited and besides additional apparatus such as a heat-treating oven and the like is required, since the secondary coating layer is orientated in a coating method (extrusion coating) which is generally used at present on the basis of the former reason, so that loss due to its orientation relaxation increases.